Alternating current motor starting apparatus and method

ABSTRACT

A starting set including an induction machine and a synchronous machine is brought up to speed before connection to the main motor that is to be started. The induction machine may have a number of poles that is a fraction, such as one-half, of the poles of the synchronous machine of the starting set so that the secondary of the wound rotor motor produces a voltage at that fraction of the line frequency at full speed. Alternatively, the induction machine and the synchronous machine of the starting set may have the same or nearly the same number of poles but the machines are concatenated to provide the desired frequency change. After the starting set is up to full speed, the main motor is connected to the secondary of the wound rotor motor and accelerates to a speed near synchronism with the secondary supply. If the main motor is a synchronous machine it is then synchronized with the secondary supply by applying field excitation. Means are then provided to increase the load on the synchronous machine of the starting set. The starting set is then disconnected from the power line so that it will begin to decelerate and, when brought to as low a speed as possible, the main motor will be running at nearly line synchronous speed and can be synchronized with the line.

United States Patent [151 3,667,015 Godwin et al. 1 May 30, 1972 [54]ALTERNATING CURRENT MOTOR [57] ABSTRACT STARTING APPARATUS AND METHOD[72] Inventors: Gurney L. Godwin, Pittsburgh; Lee A. Kilgore, Export,both of Pa.

[73] Assignee: Westinghouse Electrlc Corporation, Pittsburgh, Pa.

[22] Filed: June 18, 1970 [21} Appl. No.: 47,297

Rossman ..3 18/171 Primary Examiner-Gene Z. Rubinson Attorney-A. T.Stratton, Gordon H. Telfer and F. P. Lyle A starting set including aninduction machine and a synchronous machine is brought up to speedbefore connection to the main motor that is to be started. The inductionmachine may have a number of poles that is a fraction, such as one-half,of the poles of the synchronous machine of the starting set so that thesecondary of the wound rotor motor produces a voltage at that fractionof the line frequency at full speed. Alternatively, the synchronousmachine of the starting set may have the same or nearly the same numberof poles but the machines are concatenated to provide the desiredfrequency change. After the starting set is up to full speed, the mainmotor is connected to the secondary of the wound rotor motor andaccelerates to a speed near synchronism with the secondary supply. Ifthe main motor is a synchronous machine it is then synchronized with thesecondary supply by applying field excitation. Means are then providedto increase the load on the synchronous machine of the starting set. Thestarting set is then disconnected from the power line so that it willbegin to decelerate and, when brought to as low a speed as possible, themain motor will be running at nearly line synchronous speed and can besynchronized with the line.

6 Clains, 5 Drawing Figures induction machine and the Patented May 30,1972 2 Sheets-Sheet 2 DYNAMIC PUMP MOTOR SYNCHRONIZED PUMP MOTOR PULLEDIN AT 301-12 BRAKING H LINE SET IS STARTED SET 60 l-PUMP MOTOR STARTED II k 40" I,

60 m E 30 0 PUMP MOTOR 'SPEED I MVAR 4O 32Q z AX I E z I I IO z, I

J |/2 L J I I l O O I I TIME SECONDS FIG. 5

ALTERNATING CURRENT MOTOR STARTING APPARATUS AND METHOD BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates tostarting means and methods for AC machines.

2. Description of the Prior Art Sufficient power is often not availableto start large motors directly from a power line without excessivesystem disturbance and thus some auxiliary device is necessary to bringthe motor up to speed. In designing starting means for large machinesthere are important considerations on the size, cost and powerrequirements of any auxiliary devices required.

Large water wheel generators of approximately 100,000 horsepower, forexample, are presently used in hydroelectric generation plants. It issometimes desirable to use the generators as motors during off peakperiods to pump water back into a reservoir for subsequent use in powergeneration. Such large machines cannot be directly connected to thepower line because of the likelihood of producing undesirable systemoverloads. Therefore a starting means must be provided to bring themachine up to speed before it is connected directly to the power line.

. Standard practice in the past has been to use a starting set includingan induction motor, such as a wound rotor motor, and a synchronousgenerator on the same shaft and capable of supplying line frequency, ornearly line frequency, at full speed. The procedure is merely toconnect, electrically, the main motor to the starting generator and thenstart the wound rotor motor. The frequency starts at zero and increaseswith the speed of the set. When the wound rotor motor has been broughtup to speed, the main motor can be connected to the line and thestarting set can be shut down.

While such a starting scheme has been generally adequate it is desirableto minimize the rating and size of the machines of the starting set andit is also desirable not to have to exercise the care required in orderto avoid the negative damping characteristics of synchronous machinesoperated at low frequency.

SUMMARY OF THE INVENTION In accordance with this invention, thearrangement of starting apparatus and the starting sequence are changedfrom that which is employed in the standard procedure described above.The changes in starting apparatus and sequence permit reduction inmachine rating and size that provide substantial economies.

In accordance with this invention the starting set includes an inductionmachine, such as a wound rotor motor, and a synchronous machine. Theinduction machine may have fewer poles than the synchronous machine sothat its secondary at full speed produces a voltage at a correspondingfraction of the line frequency. The starting set is brought up to speedbefore connection to the main motor. This may be by using either one orboth of the synchronous machine and the induction machine for starting.The main drive motor is then connected to the secondary of the inductionmachine which produces voltage at a fraction of line frequency inaccordance withthe ratio of the difference of the number of poles of thesynchronous motor and the induction machine to those of the synchronousmachine of the starting set. For example, the

synchronous machine in the starting set may be an eight pole machinewhile the induction machine may be a four pole machine so that afrequency change of one-half results. The

main motor thus accelerates to half-speed using its damper windings forstarting and is synchronized to the secondary supply. The starting setis then loaded increasingly so that it is decelerated when the speed setis disconnected from the line.

As the starting set decelerates, the speed of the main motorAltematively, the two machines of the starting set may be concatenated,that is, they may be cascaded by having the stator winding of thesynchronous machine connected to the secondary of the induction machine,to provide the desired frequency change. When the machines areconcatenated the detemiining frequency ratio is the number of poles ofthe synchronous motor divided by the total number of poles of bothmachines.

In accordance with this invention the rating and size of the startingset can be reduced to about one-half of the set used in previouslyconventional practice. Also, the normal damper windings of thesynchronous machine on the starting set provides ample positive dampingso that the care previously required to prevent negative damping isunnecessary.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramof a synchronous motor and starting means therefor in accordance withthe prior art;

FIG. 2 is a schematic circuit diagram of a synchronous motor andstarting means in accordance with one embodiment DESCRIPTION OFPREFERRED EMBODIMENTS By way of further background, reference is made toFIG. 1 which shows starting means in accordance with the prior art. Anumber of main motors l0 and I2 (typically synchronous generatorsintended to be operated as motors) are connected to a power line 14through various circuit breaker means I6, 18 and 31, and a transformer20 as would ordinarily be employed. The main motors are also connectedwith a starting set 22 through individual circuit breaker means 17 and19.

The starting set 22 comprises an induction motor 23, with associatedsecondary starting resistors 24 and circuit breaker means 25 connectedto its secondary or its stator for the selective connection of theresistors 24 to the motor 23. The induction motor 23 is typically awound rotor motor because it permits connections to be made to bothprimary and secondary windings. As shown in this example, the primarywinding (to which supply line 14 is coupled) is on the rotor and thesecondary is on the stators. However, this is not necessary and theconnections may be reversed.

The starting set 22 also includes a synchronous generator 26 capable ofsupplying line frequency or nearly line frequency power at full speed.The synchronous machine 26 is on the same shaft 27 as the rotor of motor23. The stator of machine 26 is connected by line 28 and breaker means17 and 19 to the main motors l0 and 12. A main motor 10 or 12 is startedby connecting it to the starting generator 26 and then starting themotor 23 by closing breaker means 29 to power line 14. Ordinarily, atransformer 30 is between power line 14 and motor 23.

It is contemplated in this discussion of the prior standard practice, aswell as following discussion of the present invention, that individualones of the main synchronous motors 10 and 12 are separately started.The single line schematics used for illustration omit conventionaldetails of dynamoelectric machine practice such as those elementsnecessary to complete a three phase system as would ordinarily be used.Also, brushes and slip rings for connection with the rotor of the motor23, and field excitation means and other elements are not detailed.

As the starting set 22 begins, the frequency starts at zero andincreases with the speed of the set. When the starting set 22 has beenbrought up to speed the main motor 10 can be connected with the line 14by closing breaker means 31 and the starting set 22 can be shut down.

In such schemes as that shown in FIG. 1, care must be taken during thestart to avoid instability due to negative damping characteristics ofthe main machines operated at low frequency. No general solutions forthis instability exist so careful engineering design is required in eachapplication. Also, it has now been found that the machines 23 and 26 arelarger, and thus more expensive than need be the case. Eight poles forthe induction machine 23 and I poles for synchronous machine 26 aretypical of machines employed in practice.

In the exemplary embodiment of the present invention shown in FIG. 2,the main motors l0 and 12 are illustrated with a starting set 32 thatalso includes an induction machine 33 and a synchronous machine 36 butwith the secondary (stator) winding of the motor 33 connected to themain motors by line 28 and breaker means 17 and 19. The synchronousmachine 36 has appropriate loading means such as a rheostat 38 with anassociated breaker means 39 and the wound rotor motor 33 will havesuitable starting resistors 40 and associated breaker means 41. Thestarting set 32 may be brought up to speed using the synchronous machine36 alone. However, if starting time or heating is a design limit boththe wound rotor and synchronous machines 33 and 36 can be used.

Initially, the main motors l0 and 12 are not connected to the startingset 32. Only when the starting set 32 is up to speed is a main drivemotor then connected to the secondary of motor 33 which produces afraction of line frequency at full speed depending on the number ofpoles of the synchronous machine 36 and motor 33 of the starting set 32.As an example, there can be eight poles in the synchronous machine 36and four poles in the motor 33 resulting in a half line frequency atfull speed on the secondary of motor 33. The controlling factordetermining the frequency is the ratio of the difference in the numbersof poles of the two machines to the number of poles of the synchronousmachine. The main motor thus accelerates to half-speed using its damperwindings for starting and can be synchronized to the secondary supply.

At this point the rheostat 38 is set to absorb more power than is beingproduced by the synchronous machine 36 and then a breaker 42 is openedto disconnect the starting set 32 from the line so that the speed setwill begin to decelerate. The rate of deceleration is determined byadjusting the synchronous field current and the value of the brakingresistor 38.

When the speed set 32 has been brought to as low a speed as possible byapplying maximum field current and shorting the stator winding ofmachine 36, the main motor will be running at nearly synchronous speedand can be synchronized with the line. The capacitor 43 shown connectedto machine 36 through breaker means 42 and 44 can be provided to correctfor most of the reactive KVA during starting, if desired.

The synchronous speed of the 4 pole induction machine 33 coupled to the60 cycle line is 1,800 revolutions per minute but this is effectivelyoffset, initially, by the 900 revolution per minute shaft speed producedby the synchronous machine 36. The net rotating field is therefor at afrequency of 900 revolutions per minute producing a 30 cycle voltage onthe secondary of the four pole machine (4 X 900/I). When the synchronousmachine 36 is braked to a standstill, the induction machine 33 acts as astationary transformer with 60 cycle power on its primary.

It is convenient that the ratio of poles of the wound rotor motor 33 tothat of the synchronous machine 36 be about onehalf although this is notessential. It does permit the main motors to start at some fraction ofspeed on the secondary of motor 33-. In general ratios of from about 0.4to 0.6 would be preferred in the practice of this invention. In theprior art scheme a ratio of about 0.8 would be typical, requiring largermachines, more cost and higher ratings.

In FIG. 3, curves are presented showing calculated power demand versustime for starting schemes in accordance with the prior art and inaccordance with this invention. The upper curve is for a starting schemein accordance with the prior art such as that shown in FIG. 1. It showsa gradual build up in power from about 30 to near 50 MW occurs over thefirst portion 60 of the starting sequence. During this period thefrequency starts at zero and increases with the speed of the set. Theremay be an initial transient oscillation in the power demand that is notillustrated here. Continuing level power requirements over curve portion6] occur until a point after 35 seconds when the field excitation isapplied to the main motor and speed increases over curve portion 62 withconstant power demand to the point at which the speed is sufficientlyclose to the line frequency that synchronism can be at-' tained.

In the lower curve of FIG. 3, with a frequency changer (F.C.) scheme inaccordance with this invention, such as that shown in FIG. 2, there isan initial period 70 in which the starting set is energized, afterwhich, during curve portion 71, the main motor is connected to the CWsecondary with build-up in speed to half-speed, for the exampleillustrated in which the ratio of poles is 0.5, then the field isapplied and the machine accelerates to full speed during period 72 atwhich synchronism occurs. It is shown that in this calculated examplethere is substantial saving over the starting cycle in the powerrequirements of the proposed starting scheme in accordance with thisinvention compared with that of prior practice. Similar calculations canbe made to show that there are also substantial savings in reactivepower (MVAR) demand.

FIG. 4 illustrates an alternative form of the present invention whereinvarious elements are designated by reference numerals that are the sameas for corresponding elements of FIG. 2. The starting set 32 comprises awound rotor motor 33 and a synchronous generator 36 and the sequence ofoperations during the starting cycle is the same as was described inconnection with FIG. 2. However, the frequency change in the example ofFIG. 4 does not result from a difference in the number of poles of thetwo machines in the starting set but rather from the fact that twomachines, both six poles, for example, are used with their windingscascaded. The machines are mechanically connected to the same shaft 37and the secondary of the motor 33 is connected to the stator winding ofthe synchronous generator through line 50 and circuit breaker means 51.The combination of the two machines will operate at a speed equivalentto the sum of the two pole numbers for which the two machines are wound.Since the motor represents half that number of poles, its secondaryprovides a frequency change of one-half similar to the operation of theembodiment of FIG. 2. That is, with the concatenated connection it isthe ratio of the number of poles of the synchronous machine to that ofthe total of the two machines that represents the frequency change.

Going through the sequence of operations in starting, breakers 42 and 51are closed to accelerate the starting set to a speed equivalent to thesum of the poles of the motor 33 and salient pole machine 36. The fluxin the induction machine is revolving at 60 cycle speed, but the rotoris turning at 30 cycle speed. Because of this, half of the wound rotormachines input goes to the generator 36 through the common shaft 37 andhalf of its input goes to its secondary winding. By closing circuitbreaker 17, the full power is available to accelerate the main pumpmotor 10 to half-speed. In this example the main motor 10 is an 80,000horse power synchronous motor. The power bus 14 supplies 230 KV that istransformed down to 14.4 KV for the main motor 10 and to 6.9 KV for thestarting set 32.

In the next step of the starting sequence, the synchronous machine 36 isdisconnected from the motor 33 by opening breaker 51. The generator 36is then connected to a braking rheostat 38 by closing circuit breaker39. This brings the starting set down in speed and, hence, increases thefrequency output of the induction machine 33 as explained above inconnection with FIG. 2. By reducing the resistance of the brakingresistor 38 and finally short circuiting the generator with contactor52, maximum frequency and voltage output are obtained from the motor 33feeding the pump motor 10. At this point in the starting cycle thestarting sets frequency output is sufficiently close to 60 cycle speedso that the motor 10 can be easily synchronized with the line 14 byclosing its running breaker l6 and opening breaker 17.

If another 80,000 horsepower pump motor is to be started at once, thestarting set is broughtagain to the 30 cycle speed by merely openingbreaker 39 and closing breaker 51. Breaker 42 can be kept closed untilall the motors that are to be started are started.

The proposed scheme of FIG. 4 offers advantages in that it usesmachinery effectively. The torque of the starting set 32 times the 60cycle speed of the flux matches the output to the main motor 10 whereasthe actual maximum running speed is just half for the 30 cycle output.

The various embodiments of the invention employing the frequency changeconcept for starting a'large synchronous motor provide lower peripheralspeed and mechanical stress and require less volume of electricalmachinery. Speed matching is not required for synchronization.Synchronizing is simpler and much faster resulting in less duty and lessrestrictions on the set transformer 30 and associated breakers. Thispermits lower rating of breakers which have low maintenance even withfrequent operation. A liquid rheostat 38 is shown but resistors can beused and mounted under the RC. 32 and cooled by its ventilation system,thus avoiding the maintenance of electrolyte in a liquid rheostat wherethat is used for the braking resistor 38. Contactors suitable for use upto 1,000,000 mechanical operations are available. in accordance withthis invention, starting cycles of no more than about 4 minutes areusually provided for and increased torque is available for future pumpmotor starting needs.

FIG. 5 illustrates a set of curves for the operation of a system asillustrated in FIG. 4. The various curves, as labeled, show power demand(both MW and MVAR) and speed (both for the starting set and the mainpump motor) in relation to elapsed time in the starting sequence for asix pole to six pole starting set at 600 RPM.

We claim as our invention:

1. Dynamoelectric machine starting apparatus comprising:

a synchronous dynamoelectric machine;

an induction dynamoelectric machine having a commonly connected shaftwith said synchronous dynamoelectric machine and comprising therewith astarting set;

means for electrically connecting the secondary of said inductionmachine with a main motor to be started when said starting set has beenbrought to substantially full speed by connection to an AC voltagesupply;

means for effecting deceleration of said starting set; and means todisconnect said synchronous machine of said starting set from said A.C.voltage supply to decelerate said starting set while said inductionmachine is connected to said supply and to accelerate said main motor.2. The subject matter of claim 1 wherein said induction motor is a woundrotor motor having a number of poles in a ratio with the number of polesof said synchronousdynamoelectric machine that is in the range of fromabout 0.4 to about 0.6. I

3. The subject matter of claim 1 wherein: said induction motor has thesame number of poles as said synchronous machine and further comprisingmeans for connecting the secondary of said induction motor to the statorof said synchronous machine and for interrupting the connectiontherebetween to disconnect said synchronous machine from said A.C.voltage supply. 4. A method of starting an A.C. motor comprising thesteps of:

starting a starting set that comprises a synchronous machine and aninduction machine mechanically connected on a shaft with means todevelop an alternating voltage at a fraction of line voltage frequencyon the secondary winding of said induction machine; connecting a mainmotor to be started to the secondary of said induction machine onlyafter said voltage at a fraction of line voltage frequency has beendeveloped and acceleratin said main motor to a speed that is a fractionof full spee corresponding with the fraction of line voltage frequencyon said secondary;

decelerating said starting set while maintaining only said primary ofsaid induction machine connected to said line voltage supply; and

connecting said main motor with said line voltage supply after saidstarting set is near standstill.

5. The subject matter of claim 4 wherein:

the secondary winding of said induction machine is maintained withoutconnection with the stator of said synchronous machine.

6. The subject matter of claim 4 wherein:

the secondary of said induction machine is connected to the stator ofsaid synchronous machine during the starting of said starting set and isdisconnected therefrom after the starting set has reached full speed andbefore decelerating said starting set.

l I? i

1. Dynamoelectric machine starting apparatus comprising: a synchronousdynamoelectric machine; an induction dynamoelectric machine having acommonly connected shaft with said synchronous dynamoelectric machineand comprising therewith a starting set; means for electricallyconnecting the secondary of said induction machine with a main motor tobe started when said starting set has been brought to substantially fullspeed by connection to an AC voltage supply; means for effectingdeceleration of said starting set; and means to disconnect saidsynchronous machine of said starting set from said A.C. voltage supplyto decelerate said starting set while said induction machine isconnected to said supply and to accelerate said main motor.
 2. Thesubject matter of claim 1 wherein said induction motor is a wound rotormotor having a number of poles in a ratio with the number of poles ofsaid synchronous dynamoelectric machine that is in the range of fromabout 0.4 to about 0.6.
 3. The subject matter of claim 1 wherein: saidinduction motor has the same number of poles as said synchronous machineand further comprising means for connecting the secondary of saidinduction motor to the stator of said synchronous machine and forinterrupting the connection therebetween to disconnect said synchronousmachine from said A.C. voltage supply.
 4. A method of starting an A.C.motor comprising the steps of: starting a starting set that comprises asynchronous machine and an induction machine mechanically connected on ashaft with means to develop an alternating voltage at a fraction of linevoltage frequency on the secondary winding of said induction machine;connecting a main motor to be started to the secondary of said inductionmachine only after said voltage at a fraction of line voltage frequencyhas been developed and accelerating said main motor to a speed that is afraction of full speed corresponding with the fraction of line voltagefrequency on said secondary; decelerating said starting set whilemaintaining only said primary of said induction machine connected tosaid line voltage supply; and connecting said main motor with said linevoltage supply after said starting set is near stand-still.
 5. Thesubject matter of claim 4 wherein: the secondary winding of saidinduction machine is maintained without connection with the stator ofsaid synchronous machine.
 6. The subject matter of claim 4 wherein: thesecondary of said induction machine is connected to the stator of saidsynchronous machine during the starting of said starting set and isdisconnected therefrom after the starting set has reached full speed andbefore decelerating said starting set.